The transmission capacity of single-core optical fibers that are currently used is approaching its limit due to the rapidly increasing volume of optical communication traffic. In response to this situation, a multicore fiber in which multiple cores are formed in one fiber has been proposed as a means for increasing the communication capacity.
Such multicore fibers are spliced together by, for example, disposing end surfaces of the multicore fibers to face each other, applying light from one side of the multicore fibers to detect an output of the light received at the other side of the multicore fibers, and relatively moving the multicore fibers in horizontal and vertical directions to align axes at positions at which the output of the light becomes maximum (Patent Document 1).